A 1:1 electron-projection lithography machine permits the projection of an emitted pattern of low-energy electrons from a masked photocathode onto a substrate (e.g. a semiconductor wafer) as a high energy, focused, patterned beam at unity magnification, thereby permitting the transfer of the pattern to an electron sensitive material on the surface of the substrate which can subsequently be caused to selectively alter the substrate. It is possible to control the magnification by a small amount without introducing significant distortion into the projected pattern, even over a large surface such as an eight inch diameter wafer.
In using such a machine to manufacture fine geometry products such as integrated circuits having 0.1 micron features, it is generally necessary to repeat the lithography process with successive beam patterns aligned with previously created structures to an accuracy of one-fourth to one-tenth of the size of the smallest features. Thus, to use 0.1 micron features, it is desirable to align successive patterns to within 0.01 micron. The procedure used in the past for alignment of patterns in such machines has been to use two or more small structures resulting from the first photocathode pattern, which lie substantially at the ends of a diameter of the substrate, as electron detectors, and to provide a matching beam pattern on the succeeding photocathodes.
Typically an electron detector on the substrate is more sensitive to electrons than the surrounding portion of the substrate so signals can be generated which are used to align the small pattern beams with their corresponding detectors.
The prior art generally provides a system in which a beam is moved across a detector. The maximum response of the detector as the beam is moved across it denotes the condition of alignment. In order to obtain a position indication from which a useful centering signal can be derived it is necessary to provide a modulation of the beam position. Unfortunately, this also modulates the position of the entire pattern causing it to lose resolution. Therefore, the modulation must be removed after alignment, and the alignment cannot be monitored during the actual printing of the pattern. In addition, changes in the size of the detectors during intervening processing steps will reduce the sensitivity to position by widening the peak of the response.